Colicin pore-forming domains bind to Escherichia coli trimeric porins

Colicin N kills sensitive Escherichia coli cells by first binding to its trimeric receptor (OmpF) via its receptor binding domain. It then uses OmpF to translocate across the outer membrane and in the process it also needs domains II and III of the protein TolA. Recent studies have demonstrated sodium dodecyl sulfate- (SDS) dependent complex formation between trimeric porins and TolA-II. Here we demonstrate that colicin N forms similar complexes with the same trimeric porins and that this association is unexpectedly solely dependent upon the pore-forming domain (P-domain). No binding was seen with the monomeric porin OmpA. In mixtures of P-domain and TolA with OmpF porin, only binary and no ternary complexes were observed, suggesting that binding of these proteins to the porin is mutually exclusive. Pull-down assays in solution show that porin-P-domain complexes also form in the presence of outer membrane lipopolysaccharide. This indicates that an additional colicin-porin interaction may occur within the outer membrane, one that involves the colicin pore domain rather than the receptor-binding domain. This may help to explain the role of porins and TolA-II in the later stages of colicin translocation.